merge_config.sh: Allow to define config prefix

with CONFIG_ environment variable.

merge_config.sh uses CONFIG_ which is used in kernel and other projects.
There are some projects which use kconfig with different prefixes (e.g.
buildroot: BR2_ prefix). CONFIG_ variable is already used for this
purpose in kconfig binary (scripts/kconfig/lkc.h), let's use the same
rule for in merge_config.sh.

Suggested-by: Masahiro Yamada <[e-mail]>
Signed-off-by: Petr Vorel <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/kconfig/merge_config.sh

···
 	echo "  -n    use allnoconfig instead of alldefconfig"
 	echo "  -r    list redundant entries when merging fragments"
 	echo "  -O    dir to put generated output files.  Consider setting \$KCONFIG_CONFIG instead."
+	echo
+	echo "Used prefix: '$CONFIG_PREFIX'. You can redefine it with \$CONFIG_ environment variable."
 }
 
 RUNMAKE=true
 ALLTARGET=alldefconfig
 WARNREDUN=false
 OUTPUT=.
+CONFIG_PREFIX=${CONFIG_-CONFIG_}
 
 while true; do
 	case $1 in
···
 fi
 
 MERGE_LIST=$*
-SED_CONFIG_EXP="s/^\(# \)\{0,1\}\(CONFIG_[a-zA-Z0-9_]*\)[= ].*/\2/p"
+SED_CONFIG_EXP="s/^\(# \)\{0,1\}\(${CONFIG_PREFIX}[a-zA-Z0-9_]*\)[= ].*/\2/p"
+
 TMP_FILE=$(mktemp ./.tmp.config.XXXXXXXXXX)
 
 echo "Using $INITFILE as base"